steam drum repairs 3

[gibsi1]

A couple of recent tube leaks have turned into a larger headache. The leaking tubes were plugged, only to show they were still leaking in between the steam drum and tube. They could not be re-rolled due to the plugs, so it was decided to seal-weld around the tube. When pressuring up with air, before doing a hydro, the leaks had become worse. It seams as though their is a problem with the steam drum itself. I'm looking for information/guidance regarding repairs to the steam drum itself... code/approach/feasibility/etc.

 

[metengr]

We have had problems with several of our aux boiler lower drums and one aux boiler steam drum over the years with tube leaks at roll joints. We were able to seal weld repair the tubes. Several questions;

1. What type of boiler is this (power or package boiler)?
2. How many years has the boiler been in service? Cycled?

Any weld repair to the steam drum will probably be under the National Board Inspection Code (NBIC), and will be required to be performed by an R Certificate holder.

Seal welding of previously rolled tubes is effective but extreme care must be used because corrosion products can be pulled into the seal weld.

Come back with more information, I might be able to help you based on our previous experience.

 

[deanc]

They started leaking because of the heat from the welding.

You need what we old boilermakers call a "pocket". Cut the tube off past the outside(fireside) of the drum about 2-4 inches. You can do it from the outside if you can reach it or with a straight torch from inside of the tube. Leave the old tube and let it burn out.

Take a new piece of tube and orange peel one end and weld closed. Needs to be 2-3 inches past the outside of the drum.
Remove the old tube from the hole,prep and roll in with a "drum or Short roller". Make sense?

Make sure you get a qualified person/firm to do this. Seal welding is poor in this case since it is weak. A seal welded tube usually needs rerolled after welding. Bad practice for a watertube boiler. PWHT/Testing/Strength

 

[gibsi1]

We were planning to do just what you mentioned... knock out the old piece, install a new piece, re-roll, and plug. Problem is that we couldn't get the old piece out due to the old plug being driven in so tight. I'm not sure if there was just no way to get it out, or if they decided to go with the seal weld due to time constraints. I have just been notified that they went back in and re-welded. The air test held this time, and we will be performing a hydro tonight. The welds were pre-heated. We are under a serious time constaint, due to emissions testing that has already been postponed twice. The boiler has been down for 2yrs, getting a partial retube and a new low-NOx burner. Bringing it back online has been one headache after the other.

It is a package 1200# boiler that dates back to the late-60's. It was moved from one area of the site to another, years ago. It was partially retubed (d-tubes) over the past two years. I'm not sure of any retubing done before then.

 

[metengr]

The reason I was asking about the tube rolls is that some drums have serrated stub tube holes, which makes it very difficult to extract tubes because the purpose of the serration is to bite into the tube wall for a mechanical joint. We have seal welded tubes in the lower drum and in a steam drum on two aux boilers (100,000 lbs per hour) without a problem. On small boilers, you typically have to seal weld all of the tubes because the heat from welding will disturb adjacent tube rolls.

A two pass seal weld works nicely using SMAW or GTAW. I agree that the thermal stress from cycling resulted in the tube rolls breaking loose. However, seal welding is indeed effective and requires much less work.

 

[metengr]

PS.....For our aux boilers we could not reach any leaking waterwall tubes from the fireside (firebox). The aux boiler was not designed to provide access to these tubes at the lower drum or at the steam drum locations after the tubes were installed. Since we would need to have to cut our way in to even reach the leaking waterwall tubes, it was decided to explore a two pass seal weld. Seal welding was used and saved us a ton of money. The aux boiler is still in service with no leaks after 5 years.

 

[unclesyd]

If you have any other problems with leaks on the problem tubes make sure you check the drum ligaments around this location.

One problem not mentioned is when rerolling tubes there is a tendency to over roll the hole and tube and the tube will spring back making it a little harder to accomplish a seal weld.

Normally when you get a true leaker, not a weeper that is an indication that something else is going on.

 

[BJaffa]

The welding of tube ends to plugs should be a last resort. At best the plug should be welded to the protruding end of the tube only. This will provide a small flexible joint. If the tube is then also welded to the vessel wall (usually a thick section) then you have created a higher restrained joint. Cracking of the weld or cracking through the weld into the vessel ligament is likely. An old trick is to make a plug with a dead ended hole drilled into the center. This plug is designed for welding and will be more eleastic than a solid plug. Keep in mind this plug is now a code part. Blind stubs can be rolled into the tube hole instead. This process takes more time but is similar to what deanc described. We keep and inventory of spun closed nipples and plugs in various sizes for emergency use. Bjaffa

 

[davefitz]

a similar problem happened 14 yrs ago at some drums in Fort McMurray, Alberta. The drum cracking ocurrred at all repair welds becuase of several reasons, one of which is that the drum surfaces were contaminated with caustics from the boiler water treatement chemicals. In such instances, the surface must first be passivated with a weak acid solution, then grind the area to be weld-repaired, passivate second time and flush clean, grind then weld repair . If PWHT of a large section of drum is required, then the drum needs to have added some temporary supports since the permanent supports are not adequate to prevent inducing residual stresses during PWHT sagging.

 

[metengr]

I would not recommend PWHT of any in service drum. For seal welds, which are fillet welds, less then 3/4" nominal (weld throat) thickness you do not need to PWHT per Section I. Base metal thickness only applies for preheat.

Most seal welds are 3/8" or less in deposit thickness. By the way, on older B&W boilers they have rolled and seal welded their tubes in lower mud drums and steam drums from original installation.

 

[davefitz]

The main cracking in the Fort McMurray drums were longitudinal cracks more than 50% thru wall and more than 2 ft long. The amount of metal removed by grinding and repaired by welding required PWHT. The cracks were reappearing every year ( and getting larger) due to (a) did not passivate prior to weld repair and (b) did not add temp supports for PWHT .

True, for 3/8 fillet welds on rolled tubes , no PWHT required, but does require removal of caustic deposits to prevent embrittlement of weld .

 

[deanc]

This is why I love this work. Look at all the info.

In my opinion the use of "seal welding" a leaky water tube when it was not designed that way is a method of last resort.
Problems:
1.If the boiler was designed for roll joints and you seal weld for leaks,would this now be considered a strength joint? Would you need to justify it?
2.Cracks,good idea to check for same-PT or MT. Before and after.
3."Stuff" in the joint. 40 year old boiler with leaky tubes? No way to get that cleaned from the joint without tube removal. Forget GTAW,maybe you could use some P5.

So I think there are better ways. If your stuck and you must seal weld-better have your ducks in a row.

 

[rmw]

Then there was the drum in a 200K pph or so, power boiler in a southern paper mill that a boilermaker (on the night shift, and for unknown reasons) cut the ligaments out from between 4 tubes, imagine taking 4 tube holes, and making a basically rectangular hole (with rounded corners) out of.

Now, that is what I define as a headache.

That drum was weld repaired, re-drilled, and the boiler is still in service some 20 years later, so as they say, nothing is impossible.

The old boiler guru that designed and implemented the repair was a hero, and the project engineer, whose watch this happened on, got fired. And, as you guessed, so did the boilermaker.

Point being, there are always solutions available to your problem, Gibsi1.

rmw

 

[Sharik]

Seal welding boiler tubes is not such a big deal as long as, as deanc puts it, all the ducks are in a row. We do it fairly often but it is always considered a last resort.

If a tube roll is leaking and it has been rerolled and rerolled until finally the tube material is past it's modulus of elasticity, no amount of rolling is going to create a seal. The only options in this case are to replace the tube or seal weld.

If a tube has been leaking for a while and the tubesheet seating surface has been steam/water cut, no amount of rolling is going to create a seal. Either the tube has to be cut out, the tubesheet seating surface repaired and then a new tube installed or the tube is seal welded.

Even if a tube has been plugged it is still possible for the tube seal to leak. In this case the options again are to replace or seal weld.

For seal welding it is important to ensure the area to be welded is clean (the Fort McMurray case was unusual and unique. It is not normal to have such a high pH in the boiler water. There is more to this story than davefitz has written). The area should be examined with MT or PT prior to welding Preheat temperature control is essential, normally 200 F, maintained during welding and the area allowed to slow cool. The seal welds cannot be larger than 0.5" or else problems with PWHT requirements occur. The completed weld area is examined with MT or PT. The tube is then rerolled (unless there is a plug installed). All adjacent tubes are given a very light roll as well.

Seal welds from inside a drum are not considered strength welds. The welds are not holding the tubes in place and the pressure is actually working to try and force the weld into the leak.

Seal welded plugs from the outside of a drum (this happens on small drums, headers, etc.) are considered strength welds and the calculations must be performed to show the welds are sufficient to hold pressure and to hold the plugs into place.

All the above must be accepted by your friendly, neighborhood Authorized Inspector prior to the repair.

 

[GenB]

The most problems thatI have found in sealweld tubes is the dirt and rust, you should contact a corrossion engineer to provide for cleaning solution before welding,
GTAW or GMAW at low to med heat is needed, 3/16th to 1/4" toes are suffient,
if you go bigger, the HAZ is greater than the tube can hold,
light re-roll after welding,
'hope this help.ER

 

[gibsi1]

Thank you for all of the replies! More problems arise, as we speak. This particular package boiler is rated at 230,000 lb/hr. Being that it's 30+ years old, I'm not so sure that a partial retube was a great idea. It's just exposing all of the other problem areas, now. If they're wanting to continue with the seal welding of the various leaking tubes, perhaps we should just proceed in welding all of them? That would be a lot of tubes...

 

[metengr]

As mentioned in the previous posts, make sure you that you perform a good visual inspection of the steam drum ID surface to check for ligament cracks or othre problems.

If the drum is OK, and the tube ID surfaces look ok (no axial corrosion fatigue cracks that can sometimes ocurr in tube stubs), and your water leaks are at the tube roll joints, then I would seal weld all of the tubes in the steam drum. Partial seal welding (not all tubes) will not work. It is all or nothing from our experiences.

 

[athomas236]

This is an interesting thread, enjoyed reading. I remember having troubles getting expanded joints tight on a boiler in Khartoum about ten years ago.

best regards,

athomas236

 

[rmw]

gibsi1,

In a 30 plus year old boiler, you have to be suspicious of how many times it might have already been retubed, and/or re-rolled, with the potential for the tube holes in the drum having suffered from previous over rolling. Time to get out the mic's.

rmw

 

[pipewelder1999]

I have seal welded a few tubes in Recovery Boilers and Garbage Burners with few problems. As mentioned above, cleaning is needed to remove the scale from the surface around the tubes. Some of thes scale is impossible to weld to but in my experience has always been removed with a grinder.

One idea to reduce residual stress at the heat affected zone at the drum side is to run a 2 or 3 pass weld with the 1st or 2nd bead favoriing the drum side. Then weld another pass overlapping 1/2 of the prior bead. This may slightly temper the underlying bead.

Ligament craks are sometimes repaired using techniques similar to those in the NBIC. These are half bead temper techniques that I have used on cracked ligaments extend through the entire ligament Elevated preheat, tight preheat/interpass controls, bead sequence, and grinding are all used to reduce/modify the stresses from welding.

I welded a CJP PQR for this technique on 2-1/4" PLate with no strongbacks holding it down. The angular distortion was < 5 Degrees.

You received some great advice above. I hope it works well for you.



Gerald Austin
Iuka, Mississippi

http://www.weldinginspectionsvcs.com